1. Field of the Invention
This invention relates to the manufacture of a cellular synthetic resin (Class 260-2.5), with the use of agitating equipment of a kind having a fixed receptacle, and a rotating, single stirrer, rotating about a fixed vertical axis (Class 259, Sub-classes 8, 106, 108, and 111).
2. Description of the Prior Art
The manufacture of expanded polystyrene particles, particularly ones having a low density of about 2 pounds per cubic foot (33 milligrams per cubic centimeter) or lower, is well known. As commonly practiced, such manufacture involves the use of a method and apparatus as disclosed in the Rodman, Jr., U.S. Pat. No. 3,023,175. Particles of polystyrene containing, for example, about 5.8 to 7.3 weight percent of pentane or a similar suitable blowing agent, are fed to a cylindrical pre-expander vessel and agitated therein, while being treated with steam, to cause the blowing agent to volatilize and the particles to be expanded to a low density. As fed to the pre-expander, the beads have a density of about 39 pounds per cubic foot (625 milligrams per cubic centimeter). As expanded, they have a density on the order of 0.7 to 2 pounds per cubic foot (10 to 33 milligrams per cubic centimeter). Pre-expanded beads of the kind indicated above are then used in various ways known to those skilled in the art. Most typically, they are placed into a mold form, substantially filling the mold, which is closed and supplied with heat, to complete the expansion of the beads and thus form a desired object, such as a polystyrene drinking cup. In some instances, the low-density preexpanded beads are used as such, for example, as fillings for bean bags or as filler material in packaging.
Those skilled in the art have realized that higherdensity expanded-polystyrene material would be useful in various applications for which the low-density material cannot be used. For items such as shoe soles, panels for use in making furniture, and other objects now made of metal, plastic, or wood, the low-density material has inadequate compressive strength. Moreover, when attempts were made to operate the usual cylindrical pre-expander equipment of the kind generally used for making the low-density polystyrene, with the object of obtaining a higher-density product, the results were unsatisfactory. A product was obtained which contained a substantial proportion of particles expanded to the desired degree, but in admixture with particles of substantially different density, including a noticeable proportion of unexpanded beads. For practically all purposes, such a non-uniform product is undesirable. If the product were more nearly uniform, it would be possible to get a desired level of compressive strength at a somewhat lower average density, thereby saving material. Prior to this invention, a uniform product could be produced only by taking additional steps to improve the uniformity of the product, and such additional steps are unsatisfactory because they add to the cost of the product, not only because of the cost of the operation but also because of the necessity of recycling or otherwise disposing of the beads which failed to expand. If the beads fed to the pre-expander are of a very nearly uniform size, a separation by screening is possible, but with present technology, the cost of screening, both before and after, to obtain a uniform high-density product in this way would be too great to make the process commercially attractive. The same is true with regard to the possibility of separating the particles of undesirably low or high density by trajectory separation or classification.
The prior art has lacked a method and an apparatus whereby high-density pre-expanded polystyrene particles of substantially uniform density, such as 130 to 190 milligrams per cubic centimeter, or 260 to 320 milligrams per cubic centimeter, could be produced readily and conveniently.
Although the above mentioned patent, U.S. Pat. No. 3,023,175 teaches that a reaction vessel of other form "such as hemispherical or conical" could be used, it does not disclose the particular apparatus features found necessary by the applicant for the production of the desired, uniform, high-density product. It contains no appreciation or suggestion of the applicant's method for obtaining such a product conveniently.